1. Field of the Invention
The present invention relates to an ink jet apparatus for conducting printing ejection for a printing medium by ejecting ink from ejection ports as well as conducting preliminary ejection without any printing on the printing medium.
2. Description of the Related Art
Various kind of recording apparatus such as printer, copying machine, facsimile and so forth is constructed such that character and image each composed of dot pattern are printed on a printing medium such as a sheet of paper, a sheet of plastic thin plate or the like in response to character information or image information.
The recording apparatus can be classified into an ink jet system, a wire dot system, a thermal system, a laser beam system in terms of a recording system.
Among them, the ink jet system is such that each printing operation is performed by ejecting ink from an ink jet head to a printing medium, and has advantages that an image having excellent finesse can be printed at high speed, since the ink jet system is no impact system, few nosy sound is generated during printing operation, and moreover, colored image can easily be printed on the printing medium using multicolored inks. Especially, a system wherein a heat generating element is used as an ejection energy generating element for generating energy for ejecting ink therefrom has advantages that a head can be designed with small dimensions and recording can be achieved with high fineness.
With respect to an ink jet system having a heat generating element used therefor, there sometimes arises an occasion that part of dyestuff, pigment or the like is thermally decomposed on the heat generating element depending on the kind of ink, the state of usage of the head or the like, causing the decomposed substance to be adhesively deposited on the heat generating element. Due to the fact that an intensity of ink ejecting force is lowered because of the presence of adhering ingredient (hereinafter referred to as ink adhering ingredient), causing no normal ejection to be conducted, and moreover, a quantity of ink adhering ingredient differs among a plurality of heat generating elements, causing ink ejecting force to differ for each of heat generating elements, there is a danger that density fluctuation arises with formed character or image. Namely, with a conventional ink jet apparatus, there arises an occasion that the kind of available ink is restricted in order to accomplish a high quality of printing, and moreover, density fluctuation arises with character or image to be formed due to a difference of usage of the heat generating element.
Conventionally, the following measures have been proposed as a method of removing the adhesively deposited material on a heat generating element by the cavitation arising at the time when a foam caused by the heat generated by the heat generating element disappears, with respect to printing density fluctuation caused by the adhesive deposition. (1) Foams are repeatedly formed while part or the whole of ejection ports of an ink jet head is kept closed. (2) Preliminary ejection is performed using all the heat generating elements with a constant width of driving signal and a constant driving cycle. (3) Means for counting the number of usage of every heat generating element is disposed and the number of times of preliminary ejection is changed with respect to each of heat generating elements. (4) Pulse width modulation control (PWM control) is performed in conformity with a printing ejection mode of printing a printing medium by ejecting ink from ejection ports.
Among the conventional measures for removing density fluctuation of character, image or the like by adhesively depositing of ink adhering element on the heat generating element,
with the measure shown in the paragraph (1), the defoaming position can be changed depending on a manner of closing the ejecting ports, but since it is very difficult to control the manner of closing the ejecting ports, it is also difficult that ink adhering ingredient present in the wide range on the heat generating element is sufficiently removed. In addition, after processing is performed, ink adheres to the ejection port plane of the ink jet head. Thus, it is necessary to perform recovering treatment for removing the adhering ink. Since foam is generated while the ejecting ports are kept closed, bubble remaining after defoaming stays in ink jet head. Therefore, there is a danger that the staying bubble becomes a factor of incorrect printing (ejection of foam, warpage of ink ejection) unless recovering treatment is performed.
With the measure explained in the paragraph (2), since preliminary ejection is performed with a constant width of driving signal and a constant driving cycle, ink adhering ingredient present in the wide range on the heating element can not sufficiently be removed. Indeed, it is difficult to eliminate a difference in a quantity of ink adhering ingredient with respect to each of heat generating elements. For this reason, density fluctuation of character, image or the like can not sufficiently be eliminated. A problem to be solved is that a quantity of ink to be used for preliminary ejection becomes very large.
With the measure shown in the paragraph (3), a difference in a quantity of ink adhering ingredient between respective heat generating elements can be eliminated. However, another problem to be solved is that a quantity of consumption of ink to be used for preliminary ejection is still large.
The PMW controlling method shown in the paragraph (4) is practiced to modulate the pulse width corresponding to the temperature of the ink jet head, and moreover, to maintain a quantity of ink ejection at the time of printing ejection constant. As one example, the driving signal is divided into two (prepulse and main pulse) and the pause time of prepulse and main pulse is modulated. However, the PWM controlling method is a controlling method for maintaining a quantity of ink ejection constant by shortening the pause time when the ink jet head has a higher temperature and elongating the pause time when the ink jet head has a lower temperature. Therefore, since the defoaming position on the heat generating element is dependent on a quantity of ink ejection, the defoaming position does not large vary. Namely, with the PWM controlling method at the time of printing ejection, the ink adhering ingredient dispersively deposited on the heat generating element can not sufficiently be removed.